target/arm: Handle FPCR.NEP for FCVTXN (scalar)

target/arm: Handle FPCR.NEP for FCVTXN (scalar)

Unlike the other users of do_2misc_narrow_scalar(), FCVTXN (scalar)
is always double-to-single and must honour FPCR.NEP.  Implement this
directly in a trans function rather than using
do_2misc_narrow_scalar().

We still need gen_fcvtxn_sd() and the f_scalar_fcvtxn[] array for
the FCVTXN (vector) insn, so we move those down in the file to
where they are used.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>